This application is related to copending, U.S. patent application Ser. No. 869,429.
1. Field of the Invention
This invention relates generally to analog signal processors. More particularly, the present invention relates to analog signal processors which use instantaneous floating point amplifiers to amplify a fluctuating input signal to a level within preselected limits.
2. Description of the Related Art
In land seismic exploration, sound waves are commonly used to probe the earth's crust to determine the types and locations of subsurface formations. The earth's crust can be considered a transmission medium or filter whose characteristics are determined by passing sound waves through that medium. Using reflection seismic techniques, sound waves or impulses are generated at a transmission point at or near the earth's surface, and transmitted toward a subsurface target. The sound waves impinge upon subsurface reflecting boundaries and are reflected back toward the earth's surface where they are received at one or more receiving points. The received waves are detected by seismic detectors, e.g., geophones, which generate electrical signals. The electrical signals, which contain information relating to subsurface formations, are recorded in a form which permits analysis. The shape and depth of subsurface reflection boundaries and the likelihood of finding an accumulation of minerals, such as oil and gas, are discerned from the analysis by skilled interpreters.
In marine seismic exploration, operators on a boat generate sound waves from a location remote from the boat. The boat tows a cable (oftentimes referred to in the trade as "a streamer"), in which sensors are disposed in multiple sections of the cable to detect the reflection of the sound waves from the ocean floor. To obtain accurate data, geologists employ many sensors arranged in short closely spaced arrays in the marine streamer. In known systems, the signals from multiple sensors form one "channel", and each section of the cable has multiple channels. In known systems, each cable section has a controller that samples the signals from each channel.
The outputs of the channels are time-division multiplexed, amplified, digitized and recorded. In known time-division multiplexed systems, the output of each channel is sampled once per sample time, and sample times of one millisecond, two milliseconds, and four milliseconds are used.
First, the amplitude of the input sound waves varies as a function of time. Second, the amplitude of the reflected sound waves decreases with time since recording is continued after the generation of input sound waves is terminated. Therefore special amplifiers, commonly referred to as instantaneous floating point or IFP amplifiers, are used to amplify the output of the channels of a seismic array.
It is undesirable in seismic operations to utilize an amplifier having a fixed gain for two major reasons.
The gain of an IFP amplifier varies depending upon the magnitude of the input signal. The amplifier is usually designed such that the amplifier output, when sampled, is at a level within preselected limits. Typically, the IFP amplifier is designed to amplify the input signal to a level between one-half and full scale output of the amplifier.
Known instantaneous floating point amplifiers include a plurality of cascaded amplifier stages. The number of stages and the gain of each stage determine the maximum gain that the amplifier can apply to the signal presented at its input. A given stage of the amplifier is or is not used, depending upon the amount of gain that must be applied to the input signal to increase its magnitude to within the preselected limits.
Known instantaneous floating point amplifiers also include control circuitry which determines, for each input signal, those stages of amplification which are required to amplify the input signal to within the preselected limits. The control circuitry typically generates a gain word representative of which stages of the amplifier are presently being used to amplify the input signal. It is desirable to minimize the number of stages of amplification required to implement a given amplifier in order to reduce circuit complexity.
One problem with known instantaneous floating point amplifiers is that each stage of amplification has an inherent offset voltage which is amplified together with the input signal. Since it is imperative that the ultimately recorded data be as uncorrupted by noise as possible, the amplified offset voltage must be removed from the amplifier output before later processing begins. Removing the offset voltage is commonly referred to as "nulling" or "zeroing" the amplifier.
Known systems include one sample and hold amplifier per digitizing section, the seismic output from all the channels of a given section being multiplexed onto the one sample and hold. This technique causes an undesirable time shift of the data because only one channel can be sampled and held while amplification and analog-to-digital conversion is being performed on that channel's signal. A time delay exists between the multiplexing of the first signal through the floating point amplifier and the multiplexing of the last signal. Thus, the last signal amplified does not accurately represent the signal received from the hydrophones at the instant in time the first signal amplified is received.
In known systems, the signals from the seismic channels are bipolar signals, which are multiplexed onto a sample and hold or directly to a floating point amplifier. Because of the large dynamic range of signals (on the order of 110 to 120 db), a high signal level on one channel may cause that channel to be capacitively coupled to adjacent channels, giving rise to spurious signals ("cross talk") on those adjacent channels.
Another problem in known systems is that voltage error correction circuitry is required for each sampling circuit. To conserve space, designers of some known systems assume that the voltage errors occurring in each sampling circuit are identical, and thus use only one voltage error correction circuit. In such a system, the correction circuitry sums the identical error correction voltage with each channel's signal.
Yet another problem is the large amount of physical space required for the floating point amplifier. Known systems require several stages of amplification, with DC offset correction circuitry between each stage.